In a cellular telecommunications network, the geographical area covered by the network is divided into cells, with groups of one or more cells defining a service area of the network. Each of the cells contains a base station that transmits and receives control information and voice communications to and from mobile stations that are located in the coverage area of the cell. Typically, each of the cells uses a separate control channel for exchanging control information with a mobile station, and a set of choice channels for exchanging voice or data signals (traffic signals) with a mobile station. The type of control channel used in a particular system depends on the system standard. The most popular current cellular networks employ either an analog control channel (ACCH) or digital control channel (DCCH), or both an analog control channel (ACCH) and a digital control channel (DCCH).
The term ACCH was created to describe a control channel originally developed for use in a system having analog voice channels, although now an ACCH may also be used in a dual mode analog/digital system for control of digital traffic channels. Each ACCH of a system is typically assigned to a set frequency channel. An example of a network using an ACCH would be a network that operates according to the Telecommunications Industry Association/Electronic Industries Association(TIA)/(EIA) 553 analog system standard(AMPS). In AMPS approximately 21 of the 832 available frequency channels are defined for use as ACCHs.
The term DCCH is commonly used to describe a control channel used in a time division multiple access (TDMA) digital cellular system. In a TDMA system, each channel which is broadcasting at a particular frequency is divided into a plurality of time slots. Subscriber communications or control channel signals are converted to digital format and divided into short communications bursts. Each burst is tagged with an identifier, assigned a time slot, and is broadcast in an interleaved fashion with other bursts on the same frequency. At the receiving end, the identifiers are used to reconstruct the entire communication from the individual bursts. An example of a system using a DCCH would be a network operating according to the TIA/EIA IS-136 dual mode standard.
In a cellular system, a control channel must always be available for each mobile station operating within the system. This is necessary so the mobile station always has access to the control information needed to make and receive calls and to operate correctly in the system. When a mobile station is powered on at an arbitrary location within a cell of the system, the mobile station must initially locate and select a suitable control channel. As the mobile station then roams about the coverage area of the cellular system, it must be able to locate and tune to a control channel of the cells into which it roams. Control channel selection while roaming is typically performed by making measurements on control channels identified in a "neighbor cell list". The neighbor cell list, which identifies the control channels of neighboring cells, is transmit on the control channel of each cell at certain predetermined intervals. As the mobile station roams, it measures each neighbor cell control channel in the list at predetermined times, and tunes to one of the neighbor cell list control channels when the signal strength of the signal received on that control channel becomes greater than the signal strength received from the current control channel. Since the configuration of the cellular system is known, determining the correct control channels for a particular cell's neighbor cell list is a relatively straightforward task.
If a mobile station does not have, or has limited or outdated information on the identity of the nearest control channels, the process of locating a suitable control channel may present problems that do not occur when a mobile station has accurate identifying information available on known neighbor cell control channels. For example, power up of a mobile station may take place arbitrarily within any cell of the system, so initial control channel selection can also arbitrarily take place within any cell of the system. In the case of initial control channel selection, the mobile station must be capable of selecting a suitable control channel that could be one of any of the whole set of control channels in the system.
The use of DCCHs in a cellular system presents some problems in the control channel reselection process that do not commonly occur when only ACCHs are used. Since an ACCH is typically assigned to a fixed frequency in a cellular network, the frequency channels can be known at the mobile station. In an AMPS network, for example, at least one of these 21 frequency channels assigned as control channels in the AMPS standard is used in each base station of the network. It is a relatively simple task for the mobile station to scan and measure the 21 frequencies at certain times, during power up for example, to determine the control channel with the strongest received signal strength at the mobile station and to then tune to that frequency channel. That control channel would normally be the ACCH of the cell in which the mobile station is located. The mobile station does not need to know the control channel of the cell in which it was located, or the neighbor cell list of that cell, before power up in order to quickly locate a control channel. DCCHs on the other hand are defined as a certain timeslot or set of timeslots on a frequency channel in a TDMA system and, for flexibility, it is common to allow assignment of a DCCH to any one of a large number of frequency channels, possibly the whole range of frequency channels available to the base stations in the network. For example, in an IS-136 system the DCCHs assigned to the various base stations could be on any network frequency channel in the first and fourth timeslots. During control channel selection a mobile station may have to scan the whole range of frequencies in order to locate and select a DCCH. This process could take a considerble period of time.
Different techniques have been developed for DCCH selection in situations such as power up. Several solutions are included in the reference model process described in the IS-136 standard (IS-136.1-A, appendix D). In this process a mobile station initially tunes to the last used DCCH, or tunes to a strongest DCCH chosen from a number of last used DCCHs. The mobile then synchronizes to the DCCH and determines if the signal strength and service criteria are satisfied on that DCCH. If the criteria are satisfied, the mobile station then tunes to and camps on the DCCH. When camped on the DCCH the mobile station may receive messages from the system or access the system on the DCCH. If the signal strength and service criteria are not satisfied, the mobile station repeats the process on the next strongest last used DCCH, if more than one DCCH identifier is stored. If an acceptable last used DCCH cannot be found, the mobile station initiates a scan of the ACCHs of the system and tunes to the best acceptable ACCH that is found. If an ACCH is found, the mobile station searches for a DCCH pointer in the control channel information message of the ACCH. If a DCCH pointer is found the mobile station performs the IS-136 control channel selection procedure. If no DCCH pointer is found on the ACCH, or if the DCCH pointed to does not meet the control channel selection procedure criteria, the mobile station registers on the ACCH. If the mobile station is unable to find a ACCH, it performs a band scan of the entire system band. The mobile station then finds the strongest RF channel in the band. If the strongest RF channel includes a DCCH, the mobile station performs the IS-136 control channel and selection procedure and camps on the DCCH if the selection criteria are satisfied. If a DCCH is found but the selection criteria are not satisfied on the RF channel having the strongest rssi at the mobile station, the mobile station receives a neighbor cell list from the DCCH and attempts to locate an acceptable neighbor DCCH for camping. If a DCCH is not found on the current RF channel, the mobile station will look for a digital traffic channel (DTC) on the current RF channel carrying a coded digital control channel locator (CDL) information. If CDL information is found, the mobile station attempts to find a DCCH that meets reselection criteria from the CDL information, starting with the strongest DCCH. If need be, the mobile station will check all DCCHs identified in the CDL. If no DCCH is located from the strongest RF channel, the mobile station selects the next strongest RF channel and repeats the process for each RF channel, continuing until all channels of suitable signal strength have been evaluated. If no DCCH is found from the RF channel search, the mobile station will perform a search on an alternate system band, or start the process again.
There are several situations in which the IS-136 reference model process may not provide the best control channel search capability in a cellular system. One situation is when a "distant DCCH problem" occurs. A second situation is when a "single DCCH pointer problem" occurs in a cellular system that utilizes sectored cell sites.
Referring now to FIG. 1, therein is illustrated the configuration of a conventional cellular system 100 having a simulcast sectored cellsite configuration. In FIG. 1, an arbitrary geographic area be divided into a plurality of contiguous radio coverage areas C1-C10 defined by the hexagonal areas bordered by solid lines. Each cell C1-C10 includes a cell site base station B1-B10, respectively, that provides system coverage to mobile stations within the coverage area of the cell. Each of the cells C1-C10 is allocated a plurality of voice or speech channels and at least one access or control channel. Each cell site C1-C10 is shown divided into three subareas or sectors a,b,c, defined by the dotted lines radiating outward from each base station B1-B10, respectively. Although not shown, each of the base stations B1-B10 may be connected to a mobile switching center (MSC) through conventional cellular system connections. The MSC may in turn be connected to the public switched telephone network (PSTN) to allow calls to and from the PSTN. The sectors a,b,c of each cell C1-C10 each represent the area covered by one of three sectorized antennas of each base station B1-B10, respectively, that transmit control information on a DCCH. Each base station B1-B10 also include an omindirectional antenna that transmits control information on an ACCH throughout the entire area of each cell C1-C10, respectively. A mobile station operating in system 100 is capable of making and receiving calls over an air interface through one or more of base stations B1-B10 as it moves throughout the coverage area of each of cells C1-C10. The DCCH is preferred for use, and as a mobile station moves throughout system 100 the mobile station will attempt to locate a DCCH as was described previously. For explanative purposes, only the base station equipment of cellular system in 100 is shown FIG. 1.
The distant DCCH problem that occurs in a system operating according to the IS-136 reference model may occur in cellular system 100 when a mobile station is powered down in one cell of the system, and is then moved and powered up in a cell distant from the cell in which it was powered down. For example, if a mobile station has been active in cell C7, is powered down within sector a of cell C7, and is then moved into sector a of cell C6, the mobile station will begin using the last used DCCH list to attempt to locate a DCCH. If the DCCH from B7 controlling sector a of cell C7 is located and meets the minimum IS-136 strength and service aspects selection criteria, because of unusual RF propagation effects or otherwise, the mobile station will camp on that DCCH, even though the signal may be relatively weak as compared to other potential DCCHs. A DCCH of cell C6 would actually be preferred, most likely the DCCH controlling sector a of cell C6. While reselection using the neighbor cell list received from the camped on DCCH may eventually lead to a better DCCH, the mobile station may spend a relatively long time, possibly measured in seconds, camped on the DCCH of B7 controlling sector a. If a call is made or received during the time in which the mobile station is camped on the DCCH of B7 controlling sector a of cell C7, the call will be assigned a voice channel of cell C7 and might not be setup, or might be dropped, because of poor signal strength or a low carrier to interference (C/I) ratio, or the call might have to be handed off immediately.
The single DCCH pointer problem that occurs in a sectorized system operating according to the IS-136 reference model may also occur in cellular system 100. This may occur during any time in the IS-136 reference model process when a mobile station reads a DCCH pointer from an ACCH, locates a DCCH, and camps on that DCCH after determining if the DCCH meets the IS-136 strength and service aspects selection criteria. Because each sectorized cell uses three DCCHs, one for each sector, and there is only provision for one DCCH pointer on the ACCH, it may turn out that the DCCH is not the optimum choice for the mobile station. For example, if a mobile station is powered on in sector a of cell C4, initially finds no DCCH, and gets to the point in the selection process where it reads a DCCH pointer from the ACCH of cell C4, a DCCH pointer to the DCCH for sector c. Since the DCCH for sector c is radiated away from sector a, the signal strength may be relatively weak. However, it is possible that the mobile station may camp on that DCCH. Problems similar to those in the distant DCCH scenario could then occur. The pointer problem could also occur upon termination of a call between the mobile station and a base station. In the situation in which the mobile receives a call release message over a voice channel, including DCCH identifying information, it may be that a DCCH is available to the mobile station that is more preferred than the DCCH or DCCHs identified in the call release message. One situation in which this could occur would be if a mobile station released a call while moving quickly, such as by automobile, out of a first cell into a second cell and received the DCCH information from the voice or traffic channel of the first cell.
In the problem situations described above, it is possible that a second DCCH that would be more preferred for camping on then the DCCH that was actually camped on is available for use by the mobile station. For example, in the single DCCH pointer problem scenario it is obvious that the DCCH controlling sector a of cell C4 is the preferred DCCH for camping. In the distant DCCH problem scenario, a DCCH of cell C6 would probably be most preferred.
As may be appreciated from the above, it would be desirable to provide an improved method for efficient and quick DCCH selection in a cellular system, wherein the method is not significantly time consuming as compared to other methods but where the method minimizes the possibility that a first digital control channel is selected for a mobile station in a cellular system, when a more preferable second digital control channel is available for selection for the mobile station.